It is difficult to know for sure if the type of underwear you are wearing is affecting the amount of semen you are ejaculating. What we know in general is this: the testicles are located on the outside of a man’s body (in the scrotal sack) for a reason—to keep them the right temperature for sperm production (slightly cooler than the rest of the body). If you wear tight underwear all the time, it’s possible that your testicles won’t generate sperm as efficiently because they are held too close to the body and are too warm.  This is not dangerous or permanent; sperm production probably goes back to normal if you stop holding the testicles so close to the body all the time. There are also other reasons for your amount of semen to change, so it’s a good idea to talk to your health care provider about your concerns.